Why is this javascript function returning undefined ?

Question

let getLowerUpperBoundFromValue=(bound , idToValue)=>{
        // Returns the value of the variable previously generated if "value" is a variable name  or return "value" if its a number . 
        // bound :  can be a direct integer or a variable name. 
        // idToValue : contains the id to value mapping which must contain a variable whose name must be equal to 'bound' parameter if its a variable name . 

        if(isNaN(Number(bound)))
        {
            Object.entries(idToVarStatesGlobal).forEach(idStatePair=>{
                let id= idStatePair[0] , varState = idStatePair[1] ; 
                if(varState.name===bound){
                    console.log("check now Returning idTovalue[id]" , idToValue , id , idToValue[id] , Number(idToValue[id]));
                    return Number(idToValue[id]) ; 
                }
            })
        }
        else return Number(bound); 
    }

When I do a console log like this :

console.log('check now: ' , getLowerUpperBoundFromValue(varState.lowerbound , idToValue)) ;

I get the log output like this :

check now Returning idTovalue[id] {PSfCL5hBm: 69} PSfCL5hBm 69 69
inputGeneration.js:99 check now:  undefined

Why is the function returning undefined even though the Number(idTovalue[id]) evalues to a normal value 69 ?

Because that return statement is inside the forEach callback, not inside getLowerUpperBoundFromValue. Returning from an "inner" function doesn't magically return from the outer function. Simple example: function outer() { function inner() { return 'inner'; }; inner(); return 'outer'; }; outer(); returns "outer". — Felix Kling
– Felix Kling, Commented Nov 28, 2018 at 19:00
@Solo: forEach doesn't return anything, so that wouldn't help much either. — Felix Kling
– Felix Kling, Commented Nov 28, 2018 at 19:01
forEach is the wrong thing to use to find an index/entry in an array. You should use find() — epascarello
– epascarello, Commented Nov 28, 2018 at 19:04

Smarticles101 · Accepted Answer · 2018-11-28 19:08:14Z

Nothing is returning because the forEach callback is a seperate method. I removed the call to .forEach and replaced it with a for of loop which maintains scope for the return

let getLowerUpperBoundFromValue=(bound , idToValue)=>{
        // Returns the value of the variable previously generated if "value" is a variable name  or return "value" if its a number . 
        // bound :  can be a direct integer or a variable name. 
        // idToValue : contains the id to value mapping which must contain a variable whose name must be equal to 'bound' parameter if its a variable name . 

        if(isNaN(Number(bound)))
        {
            for (let idStatePair of Object.entries(idToVarStatesGlobal)) {
                let id= idStatePair[0] , varState = idStatePair[1] ; 
                if(varState.name===bound){
                    console.log("check now Returning idTovalue[id]" , idToValue , id , idToValue[id] , Number(idToValue[id]));
                    return Number(idToValue[id]) ; 
                }
            }
        }
        else return Number(bound); 
    }

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